This laboratory has presented evidence that bacteriophage T4 DNA synthesis is catalyzed by a complex in which ribonucleotides are channeled to deoxyribonucleotides and then to DNA via an integrated series of reactions. Enzymes synthesizing deoxyribonucleotides thus have a second direct role in the DNA replication process. The system has been followed in vivo by the use of isotopic compounds, and the study has been facilitated by the use of mutants blocked at various steps in T4 DNA synthesis.